The present invention relates to interference in radio communication networks, and more specifically to determinations of intra- and inter-cell interference and downlink channel quality in radio communication networks.
Radio communication networks are well known. For any practical embodiment, it is important that such a network be designed to operate in accordance with at least one of a number of well-known standards. A recent set of such standards is known as the Universal Mobile Telecommunications System (UMTS). UMTS is a part of the International Telecommunications Union's “IMT-2000” vision of a global family of “third-generation” (3G) mobile communications networks. It is expected that UMTS will play a key role in creating the future mass market for high-quality wireless multimedia communications that will approach 2 billion users worldwide by the year 2010.
In radio communication networks, such as those which operate in accordance with UMTS, it is desirable to obtain a measure of the downlink channel quality. It will be recognized by those skilled in the art that the downlink channel is a channel for transmissions from base stations to mobile stations. One known method for obtaining a measure of the downlink channel quality is for a mobile station to measure the block error rate (BLER) of transmissions received from a base station. In order to obtain a satisfactory BLER estimation, more than 100 error events must occur. Accordingly, with a BLER of 1%, the BLER estimation will require 1,000 blocks to provide a satisfactory BLER estimation.
Recently, investigations have been made into achieving a relatively fast mechanism for obtaining a measure of downlink channel quality. Specifically, it would be desirable to obtain a measure of the downlink channel quality within 100–300 milliseconds. However, the time required to collect the statistics to obtain a satisfactory BLER, (1000 blocks in the example above), does not allow the use of BLER to obtain a determination of downlink channel quality within the desired range of 100–300 milliseconds.
One mechanism which has been proposed for achieving a relatively fast measurement of downlink channel quality is the use of signal-to-interference (SIR) measurements on each radio link. However, there are many deficiencies to the use of SIR measurements in networks such as those which operate according to UMTS. For example, SIR measurements are not currently included in the signaling between mobile stations and base stations. Accordingly, mobile stations which are release 99 and release 4 terminals will not be able to perform these measurements. Furthermore, this solution adds yet another measurement that the mobile stations would have to perform on the received dedicated channel.
Another deficiency of SIR measurements is that they are degraded by the bias at low SIR levels. FIGS. 1a–1c illustrate this bias. In FIGS. 1a–1c Es represents the symbol energy while Ioc represents the interference energy to the carrier signal. FIGS. 1a, 1b and 1c are graphs showing ideal simulations of the absolute performance of SIR measurements on dedicated channels received on an AWGN channel. In FIGS. 1a and 1b the simulations are illustrated with 1 slot and 300 slots averaging, while FIG. 1c illustrates only 300 slots averaging. Under these conditions, the intra-cell interference does not interfere with the dedicated channel and is therefore not used as a parameter. The symbol energy, Es, is 10*log(SF)*Ec. For this 12.2 kbps test case, Ec=Es−24 dB. As illustrated in FIG. 1a, the performance of the measurement was degraded by the bias at low SIR levels. As illustrated in FIG. 1b, the variance of the estimated SIR increases as the SIR decreases. The variance of the measurement is, however, very small after averaging over 200 ms, i.e., 300 slots, as illustrated by the dashed curve in FIG. 1b which essentially is on the x-axis. With a linearized estimate, the variance will increase. FIG. 1c illustrates that for the case of 300 slot averaging, as the SIR decreases the block error rate will increase. Accordingly, it can be seen that using SIR measurements to achieve a fast measurement of the downlink channel quality will provide unacceptable results at low SIR levels. Therefore, there is a need for a better solution to the problem of providing SIR measurements on the downlink channel.
Although, as described above, there are known mechanisms for obtaining a measure of downlink channel quality, most known mechanisms do not provide the downlink channel quality within the 100–300 millisecond range required to achieve the above-identified benefits of a fast measure of downlink channel quality. For example, as discussed above, in order to measure the downlink channel quality using the block error rate (BLER), a mobile station must receive many blocks from a base station, 1,000 blocks with a 1% BLER, to obtain a satisfactory BLER estimation. This process will take longer than the 100–300 millisecond range required to achieve the above-identified benefits of a fast measure of downlink channel quality. Other mechanisms, such as employing SIR measurements, to estimate downlink channel quality are not backward compatible with existing mobile stations and exhibit a bias at low SIR levels. Accordingly, it would be desirable to provide methods and apparatus to achieve a measure of downlink channel quality which is relatively fast, i.e., within 100–300 milliseconds, that is backward compatible with existing mobile stations and which performs well under a variety of SIR levels.